1. Field of the Invention
The present invention generally relates to high speed data communication between digital circuits that require the driver (and/or receiver) circuits be terminated with an impedance that matches the characteristic impedance of the transmission path linking the transmitter and receiver. The present invention discloses a resistor based driver that maintains a constant output impedance while allowing the control circuits to switch the output drive to the desired level.
2. Description of the Related Art
Existing drivers use current sources or voltage amplifiers (which require a certain amount of voltage overhead) to insure that the driver remains in the linear operating region of the current source or voltage amplifier. This voltage overhead increases the driver's power dissipation and decreases the maximum signal amplitude.
For example, FIGS. 1 and 2 represent conventional driver circuits that utilize current sources and/or voltage amplifiers. More specifically, FIG. 1 illustrates a current mode driver 100 that has two power supply inputs (a voltage high input VH and a voltage low input VL) and n+1 digital control inputs that control the amplitude of the output signal (VOUT). A series of current sources 101 are connected between VL and VH through switches 102 and terminating resistors 110, 111. The output impedance of the driver, Z0 is equal to the value of terminating resistor 111. The current sources are selectively switched (using switches 102) between terminating resistors 110, 111 to control the amplitude of the output signal. The maximum amplitude of the output voltage is limited by the operating range (or compliance voltage range) of the current sources and the voltage drop across the switches (Vout (max)=(VH−VL)−(V101(min)+V102)). The additional voltage needed for proper operation of the current sources (V101(min)) and switches (V102) results in increased power dissipation and limits the maximum output signal for any given VH and VL. Item 120 represents the transmission line and item 121 represents its terminating impedance 121.
FIG. 2 illustrates a conventional voltage mode driver 200 that includes an amplifier 201 and a terminating resistor 202. The amplifier has two power supply inputs (VH and VL) and an analog control input signal Vin that controls the amplitude of the output voltage VOUT. The output impedance of the driver, Z0 is equal to the value of terminating resistor 202. A transistor level implementation of the amplifier 201 consists of a control block 205, and two driving transistors 203, 204. Since the amplifier must remain in its linear operating region to function as an amplifier, its output voltage range must be less than the supply voltages (i.e., VAMP(max)=VH−V203(min) and VAMP(min)=VL+V204(min)). The maximum signal amplitude is therefore VOUT(max)=VAMP(max)−VAMP(min)=VH−VL−(V203(min)+V204(min)). This occurs because the driving transistor 203 loses its ability to control the output voltage as VH−VAMP approaches zero and transistor 203 loses its ability to control the output voltage as VAMP−VL approaches zero. The difference between the output range of the amplifier (VAMP(max)−VAMP(min)) and the power supply range (VH−VL) is the driver's voltage overhead (V203(min)+V204(min)) that results in increase power dissipation and limits the signal amplitude. In addition, the bias currents that are necessary for the control circuits 205 also add additional power consumption. Driver 200 maintains a specific output impedance by inserting a terminating resistor 202 in series with the amplifier 201. An amplifier has a low output impedance. Problems associated with these structures are power dissipation and limitations on maximum signal amplitude.